WEST, Texas — The blast was so powerful that the U.S. Geological Survey registered it as a 2.1-magnitude earthquake. It reduced an apartment complex to a charred skeleton, leveled homes in a five-block radius and burned with such intensity that railroad tracks were fused together.

It killed an unknown number of people and injured as many as 180. Volunteer firefighters were missing. Residents of a nursing home were pulled from debris and rushed to hospitals.

By Thursday evening, one day after a fertilizer plant caught fire and exploded, no one among the hundreds of local, state and federal officials and first-responders who converged on this town north of Waco was certain about the cause.

They only knew its effect.

"There are homes that are no longer homes," said Sgt. W. Patrick Swanton, a spokesman for the Waco Police Department, who toured the streets near the plant at 10 p.m. Wednesday, about two hours after the blast.

A day later, it took the descriptive skills of survivors and emergency workers to get a sense of the true scope of the devastation — the authorities prohibited anyone not involved in the search-and-rescue effort from entering the area around the plant and even put a no-fly zone restriction on the airspace overhead.

But what became clear was that the explosion had destroyed a significant piece of a small town in the center of Texas, damaging up to 75 homes and setting off an extensive search for survivors in the rubble of the plant and the surrounding buildings.

The smoke that wafted over them seemed out of place in this green, cattle-rich area locally known as the boyhood backyard of country singer Willie Nelson.

A spokesman for the FBI in San Antonio said Thursday there had been no indication of criminal activity in the West plant explosion.

The authorities said they think at least five people and perhaps as many as 15 had died, though they said that number could rise. Three to five members of the West Volunteer Fire Department and those of other towns remained missing.

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They had responded to a fire that broke out at the plant about 7:30 p.m. Wednesday and were fighting the blaze when the blast occurred about 7:50 p.m.

"The explosion came very quickly," Swanton said. "They knew the threat. They knew the seriousness of the situation they were in. They immediately started moving to an evacuation process, absolutely doing the right thing to try and get people out of harm's way."

Law enforcement officials said they had not determined the cause of either the fire or the explosion, and were focusing on the search for survivors.

The blast occurred two days before the anniversary of the bombing of the Alfred P. Murrah Federal Building in Oklahoma City, an attack set off by explosives made from fertilizer that killed 168 people April 19, 1995. And it happened two days after bombs exploded at the finish line at the Boston Marathon.

The White House issued a statement from President Barack Obama, in which he pledged that the Federal Emergency Management Agency and other federal agencies would join state and local efforts "to make sure there are no unmet needs as search and rescue and response operations continue."

Gov. Rick Perry called the explosion "a truly nightmare scenario" and said that information about death and injury is "very preliminary." But he said that because of the size of West — the population is 2,700 — "this tragedy has most likely hit every family."

The plant, the West Chemical and Fertilizer Co., which is owned by Adair Grain Inc., had only nine employees. It did not manufacture any products, but instead stored and sold agricultural chemicals and fertilizer to farmers. The company stored substantial amounts of chemicals used as commercial fertilizers that can become explosive under proper conditions: anhydrous ammonia and ammonium nitrate.

Anhydrous ammonia is stored as a liquid in pressurized tanks, and farmers inject it into the soil, where it vaporizes into a colorless, corrosive gas. Ammonium nitrate is usually sold in granular form and was used in the Oklahoma City bombing.

A filing late last year with the Environmental Protection Agency stated that the company stored 540,000 pounds of ammonium nitrate on the site and 110,000 pounds of anhydrous ammonia.

Records kept by the federal Occupational Safety and Health Administration show that the last time the agency inspected the plant was 28 years ago. In that inspection, dated Feb. 13, 1985, the agency found five "serious" violations, including ones involving improper storage and handling of anhydrous ammonia and improper respiratory protection for workers. The agency imposed a $30 penalty on the company.

An agency spokesman said the plant was not included in its so-called National Emphasis Plan for inspections because the facility did not produce explosives, had no major prior accidents and the EPA did not rate it as a major risk.

Zak Covar, executive director of the Texas Commission on Environmental Quality, said the company has "an average compliance history," with one air-quality complaint registered.

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